The electrodeionization apparatus used for producing the deionized water is employed in various fields including the semiconductor manufacturing plants, the liquid crystal display manufacturing plants, the food processing industry, the electric power plants, household equipments, laboratories and the like. FIG. 6 shows a conventional electrodeionization apparatus in which a plurality of anion exchange membranes 13 and a plurality of cation exchange membranes 14 are alternately arranged between electrodes (anode 11, cathode 12) in such a manner as to alternately form concentrating compartments 15 and desalting compartments 16, and the desalting compartments 16 are filled with ion exchangers 10. In FIG. 6, the reference numeral 17 denotes an anodic compartment and the numeral 18 denotes a cathodic compartment.
A part of the concentrated water flown out from the concentrating compartment 15 is fed into the anodic compartments 17 and the desalting compartments 18.
In an electrodeionization apparatus, H+ ions and OH− ions are formed by dissociation of the water to continuously regenerate the ion exchangers filled in the desalting compartments so that the electrodeionization apparatus can efficiently deionize the water without regeneration with using agents which are employed in a conventional ion exchange apparatus which is widely used for desalting water. An electrodeionization apparatus produces highly pure water continuously, so that it is employed widely in a pure water producing apparatus or the like.
Generally, when the electrodeionization apparatus is applied with the electrical current exceeding the critical current density to deionize, OH− and H+ are formed by water dissociation as described above to carry the electric charge. H+ ion has mobility of 349.7 cm2 Ω−1eq−1, which is very large in comparison with that of the other ions (30 to 70 cm2 Ω−1eq−1). Therefore, particularly when the diluting compartment has a large thickness W, the difference of the mobilities between H+ and OH− is increased so that H+ tends to be quickly discharged to the concentrating compartments and OH− tends to remain in the desalting compartment. Furthermore, Na+ and K+ also tend to remain in the desalting compartments because these are monovalent and H+ ion carries the electrons, while the multi-valent cations and anions including Ca2+, Mg2+ are discharged to the concentrating compartments with relative ease. As the result, the product water tends to include monovalent alkali such as NaOH and KOH so that the product water (deionized water) becomes to contain Na ions at a high concentration (Na leak phenomenon).
An electrodeionization apparatus in which a desalting compartment is provided with vertical partition ribs for dividing the desalting compartment into cells being long in the vertical direction is disclosed in JP4-72567B. According to this electrodeionization apparatus having the desalting compartment divided into long cells by ribs in which ion exchange resins are filled respectively, the channelizing phenomenon where the flow of water from the inlet to the outlet of the desalting compartment is partially one-sided is prevented and the ion exchange resins in the desalting compartment are prevented from being compressed or moved.
The desalting compartments are filled with an equal amount of anion exchange resins and cation exchange resins so that the volume ratio of the anion exchange resins is 50 vol. %.
In the electrodeionization apparatus of JP4-72567B, the number of the cells is limited because the cells are formed by dividing the desalting compartment in the vertical direction. That is a large number of cells can not be formed in the apparatus. Further, the flow of the water in a lateral direction is blocked by the ribs, so that the contact efficiency between the water and the ion exchange resins is poor. In addition, the ion exchange resins are compressed at lower portions of the cells so that the cells have a vacancy at upper portions thereof, whereby the rate of filling the ion exchange resins tends to be poor.
The applicant disclosed, in JP2001-25647A, an electrodeionization apparatus which overcomes problems described above, which has high contact efficiency between water and ion exchanger, and which has high filling density of the ion exchanger. The applicant also disclosed, in JP2003-126862A, that this type of electrodeionization apparatus is improved in a desalting-efficiency when it has 60 to 80 vol. % of a volume ratio of anion exchange resins in the desalting compartments.
These electrodeionization apparatuses have desalting compartments, each of which is divided into a plurality of cells by a partition member, and ion exchange resins are filled in the respective cells. At least a part of the partition member facing the cell is inclined relative to an average flow direction of the water in the desalting compartment. The inclined part of the partition member allows permeation of the water, but prevents the ion exchanger to pass therethrough. Therefore, at least a part of the water flowing into the desalting compartment should flow obliquely relative to the average flow direction of water, so that the water is dispersed overall the desalting compartment, thereby improving the contact efficiency between water and ion exchanger and improving the deionization property.
When a plurality of cells are arranged along the membrane surface both in the average flow direction of water and a direction perpendicular to the average flow direction, (for example, when the apparatus has a large number of cells which are arranged vertically and laterally), the contact efficiency between water and ion exchanger becomes extremely high. Since the height of each cell is low, the ion exchanger is scarcely compressed. A vacancy is not formed at an upper portion in the cell, and the cell is filled evenly with the ion exchanger.
Generally, in an electrodeionization apparatus, ions contained in water to be treated move from a desalting compartment to a concentrating compartment depending on a potential difference between electrodes. Therefore, weak electrolytes including carbonic acid, silica and the like are hard to be removed from the water to be treated. For example, in an electrodeionization apparatus in which the anion exchange resin ratio is 50 volume % as described in JP4-72567B, the removal rate of silica is as low as about 70 to 90%.
JP2003-126862A above referred discloses the electrodeionization apparatus in which partition members are provided in a desalting compartment to divide the desalting compartment into a plurality of cells surrounded by the partition members and a cation exchange membrane and an anion exchange membrane, and the cells are filled with a mixture containing an anion exchange resin and a cation exchange resin at a mixing ratio of the anion exchange resin to the total amount of the anion exchange resin and the cation exchange resin of 60 to 80 volume %, in order to improve the removal rate of weak electrolytes.
The ratio of the anion exchange resin is made high in this Japanese publication due to the following reason:
Carbonic acid (CO2) as a weak electrolyte changes to bicarbonate ion by ionization reaction with hydroxide ion (OH−) (CO2+OH−→HCO3−).
The bicarbonate ion moves from the desalting compartment to the concentrating compartment through the anion exchange membrane. Therefore, it is important for removal of carbonic acid first to promote the ionization reaction, and secondly to improve the mobility of the bicarbonate ion. In order to promote the ionization reaction of carbonic acid (formation of bicarbonate ion), OH− ion is required to be fed, and it is brought by dissociation of water (H2O→H++OH−).
The water dissociates between the ion exchange resins and between the ion exchange resin and the ion exchange membrane. The hydrogen ion and the hydroxide ion produced between the ion exchange resins have a short lifetime because they associate with each other again in the desalting compartment. Therefore, the OH− ions produced between the ion exchange resin and the ion exchange membrane, especially between the cation exchange membrane and anion exchange membrane are effective for ionizing carbonic acid. As the amount ratio of the anion exchange resin is made higher, the contact ratio of the anion exchange resin to the cation exchange membrane becomes higher, thereby the amount of the OH− ions to be produced increases. As a result, the ionization-reaction of carbonic acid is promoted.
As the amount ratio of the anion exchange resin increases, the amount of OH− ions to be produced also increases, but the removing rate of Na+ ions is deteriorated because the amount of H+ ions decreases, thereby the treated water is deteriorated in resistivity.
In JP2003-126862A, Na leakage is prevented by adopting the structure of the electrodeionization apparatus in JP2001-25647A above referred (in which a desalting compartment is divided into a plurality of cells), which is superior in the deionizing property.    [Patent Reference 1] JP4-72567B    [Patent Reference 2] JP2001-25647A    [Patent Reference 3] JP2003-126862A
As the concentration of carbonic acid increases in water, the equivalent electrical conductance also increases in accordance thereto whereby the current density required for deionization becomes higher than that in conventional apparatuses. When the anion exchange resin is filled in a desalting compartment in a large amount ratio, a voltage applied between the electrodes should be higher than that when the anion exchange resin is filled therein with an amount ratio of 50%, in order to make the current density high. When the anion exchange resin is filled in the desalting compartment in an increased amount ratio, the cation resin is filled therein in a decreased amount ratio. When the volume ratio of the anion exchange resin in the desalting compartment is increased from 60% to 70%, pathway of ions via the anion exchange resin increases by about three times, but pathway of ions via the cation exchange resin decreases to one tenth due to reduction of cation exchange resin ratio from 40% to 30%.
As described above, as the current density becomes higher, the amount of H+ ions produced by the dissociation of water increases. Both Na+ ions and H+ ions move competitively via the cation exchange resin having the reduced pathway, but the pathway is occupied by H+ ions with priority due to a very large mobility thereof. As a result thereof, Na+ ions become hard to move the pathway, electrical resistance increases and voltage applied to the electrodes is increased.
In case that anion exchange resin is filled in a desalting compartment merely in a large amount ratio like as JP2003-126862, the current density is not increased, resulting that carbonic acid can not be removed sufficiently and that resistivity of treated water increases. Since the rise of voltage leads to the rise of electric power consumption, it is uneconomical.